Instead of racy documents, users who accept and open infected files get a comical photo of a roasted chicken with a bikini tan line. In the background, Bropia-F installs a variant of the infamous Agobot (AKA Pahtbot or Rbot) worm, opening a backdoor on infected systems. The bot can then be used to collect system information, log keystrokes and relay spam.

"Many corporations have been blocking use of instant messenger programs for employee productivity reasons, and now may have good cause to do so for security reasons as well," said Joe Hartmann, senior virus researcher at anti-virus firm Trend Micro. "With the popularity of instant messengers, it may be the home users who are most at risk - this kind of worm uses humour to make people forget that they are being infected and backdoors are being opened into their systems."

Bropia-F arrives in a file about 184 KB in size. It tries to spread to other MSN Messenger users by sending a copy of itself under one of these filenames: bedroom-thongs.pif, hot.pif, lmao.pif, lol.scr, naked_drunk.pif, new_webcam.pif, rofl.pif, underware. pif and webcam.pif.

Sightings of the worm have been reported in Taiwan, China, Korea, Costa Rica and the US since it began spreading earlier this week. Anti-virus firms rate the worm as a medium risk.

Users are also advised to apply the principles of safe computing and to exercise caution in the programs they allow to self-install on their PCs. Windows users are also advised to update their antivirus signature files to detect the worm, just in case. ®